This invention relates generally to an improved chemical vapor deposition (CVD) apparatus for the nucleation and growth of diamond crystals and more particularly concerns a CVD apparatus having a preheater for heating the stream of feedstock as it enters the apparatus.
Various methods are known for the synthetic production of diamond. One method utilizes CVD techniques. In such a process, a gas mixture of hydrogen and a hydrocarbon such as methane is used as feedstock. The gas mixture is heated to the appropriate dissociation temperature whereupon the hydrogen is converted to atomic hydrogen and the original hydrocarbon is converted to various intermediate hydrocarbon radicals (such as CH.sub.3, CH.sub.2, CH, etc. when methane is used). A substrate is provided and positioned so as to be impinged with hydrogen atom and hydrocarbon radical species fluxes. The substrate is maintained at a temperature which promotes nucleation and growth of diamond from the hydrocarbon radicals deposited thereon. To obtain high quality diamond with uniform thickness, it is necessary that the temperature of the substrate and the fluxes of hydrogen atoms and hydrocarbon species impinging the substrate be kept as uniform as possible.
FIGS. 1A and 1B show a conventional apparatus 10, (referred to herein as a "filament reactor") which uses CVD techniques to produce diamond. The filament reactor 10 of FIGS. 1A and 1B comprises a reaction chamber 12 having a gas inlet 14 and an exhaust port 16. Two substrates 18 are located in the reaction chamber 12 in a spaced apart, parallel relationship. A plurality of heating filaments 20 is positioned in the space between the two substrates 18. The filaments 20 are connected between a pair of electrodes 22 which in turn are connected to a power source (not shown). When activated, the power source generates an electrical current through the electrodes and the filaments causing the filaments to produce heat. In practice, a hydrogen/hydrocarbon gas mixture is caused to flow into the reactor chamber 12 via gas inlet 14 and to flow between the two substrates 18. The filaments 20 are provided with an electrical current from the power source sufficient to heat the filaments 20 to the necessary temperature.
The filaments 20 must heat both the gas mixture to dissociation temperatures and the substrates to the narrow band of temperatures conducive to the nucleation and growth of diamond crystals. The filament temperatures must be controlled with a high degree of precision to meet these two objectives given the complexities of convective and diffusive heat transport, radiation, exothermic hydrogen atom recombination and large density variations. This is particularly true for conventional filament reactors because the gas mixture enters the reaction chamber 12 at room temperature, which is considerably cooler than the temperature of the filaments.
The relatively cool inlet temperature of the gas mixture creates other difficulties as well. As the cool gas mixture passes over the filaments 20 (from left to right as seen in FIG. 1A) to be heated, the mixture absorbs more heat from the forward (left most) filaments than from the rear (right most) filaments. Since less heat is transferred to the gas from the rear filaments, the rear portions of the substrates 18 are heated more than the forward portions, thus creating temperature gradients across the substrates. These temperature gradients ultimately lead to non-uniform thicknesses in the diamond produced on the substrates, often to the point of adversely affecting the commercial value of the product.
Another problem arises due to the large difference between the inlet and filament temperatures. When the cool gas mixture impinges on the filaments 20 between the two substrates 18, it is rapidly heated from room temperature to a temperature on the order of 2000.degree. C. This rapid temperature elevation causes a large volumetric expansion in the space between the two substrates. The volumetric expansion creates a pressure drop which tends to cause the flow of gas mixture to by-pass the region between the substrates, thus depriving the region of fresh hydrogen/hydrocarbon gas mixture. This deprivation causes inconsistent hydrogen atom and hydrocarbon species concentrations which hamper diamond production and quality.